Today's safety helmets, such as those for use by bicyclists and skateboarders, are designed to meet linear head acceleration thresholds to avoid risk of skull fracture and focal brain injury in idealized vertical falls. They are, however, ineffective in reducing the risk of diffuse brain injuries (e.g. diffuse axonal injury) secondary to rotational motion generated during more common oblique falls.
Cycling and skateboarding are increasingly popular (and visible) forms of recreation and modes of transportation. Sadly, however, cycling fatalities have increased at a faster rate than increases in the number of cyclists [5, 6]. According to the National Highway and Traffic Safety Administration (NHTSA), 677 cyclists were killed, and an additional 48,000 were injured in motor vehicle traffic crashes in the United States in 2011 [6]. Head injuries (HI) are often the most frequent and severe cycling injuries, contributing to 66% of hospital admissions and 75% of deaths [7, 8].
Properly fitted helmets are largely recommended as “the single-most effective way to prevent head injury” [7], and several meta-analysis studies indicate that contemporary helmets effectively prevent head, facial, and brain injuries for cyclists of all ages, involved in all types of crashes [7, 9-11]. The actual efficacy of helmets is, however, a subject of heated debate. Critics have questioned not only the weaknesses in epidemiology literature (e.g. selection bias, miscued interpretations), but also the suboptimal and inadequate design of conventional helmets [9, 12-15].
Most (>60%) cycling-related head injuries (HI) are caused during oblique impacts (typically, body impact angle <30° to the ground/car), which generate a combination of linear and (relatively larger) rotational forces [2, 8, 12, 13, 16-18]. The shear modulus of brain tissue is 5-6 orders of magnitude less than the bulk modulus, and the brain is therefore significantly more sensitive to rotation-induced shear loading [2]. Notably, the relative rotation of the brain to the skull induces large shear strains in the brain, and is a well-recognized cause of a range of traumatic brain injuries (TBI), even in the absence of a direct head impact [2, 3, 12, 13, 16, 18]. However, mandatory helmet test standards, such as BS EN 1078:1997 [1], assess integrity and shock absorption capacity only through perpendicular impact (drop) tests, and assume that linear head acceleration is a sufficient indicator for HI thresholds. They do not take into account head kinematics or impact direction (and therefore the contribution of rotational acceleration), the latter of which is likely to reduce safety thresholds [13, 16]. In essence, conventional helmets are neither designed nor tested to mitigate the more frequent and severe oblique impact-induced HI, and there is evidence that the added weight of such ineffective helmets may even increase the risk of TBI [4, 12-14, 19]. The rising cases of cycling-related TBI, in spite of increased rates of helmet use, are therefore, not surprising [13].
The applicants have identified the pressing need (and opportunity) to develop a safer, advanced, ‘eco-structural’ bicycle helmet, which incorporates dedicated mechanisms to protect against angular acceleration and consequent injuries to the brain.
With that goal in mind, the applicants have developed a helmet that meets all the safety requirements of current standards (e.g. peak linear head acceleration <250 g-300 g, for linear (drop) velocities ranging between 4.4-6.7 m/s [1, 2]), and will specifically incorporate novel, dedicated mechanisms to mitigate angular head acceleration (e.g. peak angular head acceleration <8-10 krad··s−2, for rotational velocity <70-100 rad··s−1 [3, 4]). The helmet is light-weight (250-350 g) and comfortable (e.g. provide adequate ventilation). While functionality (i.e. prevention and mitigation of head injury) is prime, sustainability is an ever-important theme. Therefore, the helmet employs eco-friendly (i.e. bio-sourced), if not fully natural and/or biodegradable, materials as sustainable materials solutions.